Baking powder does go bad even when unopened.
Unopened baking powder will last for up to 18 months before it starts to go bad.
After 18 months the baking powder will start losing it's potency and won't work well in baked goods.
Opened baking powder is good for around 6 months.
Expired baking soda or expired baking powder will no longer react properly in the recipes.
This means that baked goods such as cakes won't rise, cookies won't spread, and baked goods will end up tough and dense.
You *can* still use inactive baking soda or baking powder to scrub things (the abrasive qualities will still work).
To check if your baking powder is still active, pour 1/4 cup boiling water over 1/2 tsp baking powder.
If it bubbles, it's still good. If not, it's no good!
If it foams when you pour over boiling water, the baking powder is active!